Early construction of disposable hygiene products consisted simply of a topsheet, a pulp or tissue wadding core, and an impermeable backsheet. Significant developments in more recent years have included the move to incorporating superabsorbent materials in the absorbent core. However, devices having only a topsheet, a pulp and superabsorbent core, and a backsheet are outperformed by structures which include a transfer layer (fluid management layer) between the topsheet and the core. The transfer layer might be a fabric layer, a pulp/fiber composite, or even a foam layer. The transfer layer functions to provide a surge capacity for large voids to prevent overflow leakage, to provide capillary suction to draw liquid from the topsheet into the absorbent core, and to retard or inhibit liquid from coming back up through the topsheet and onto the wearer's skin.
Therefore, the challenge is to design each individual transfer layer and to optimize the placement sequence of multiple transfer layers in the product. The manufacturer of a nonwoven material has limited opportunity to affect the absorbency of the material. However, the manufacturer can control the average pore size of the nonwoven material. The pore size is a characteristic of the material that helps to determine its ability to wick fluid and to rapidly transfer fluid, i.e., the permeability of the material.
Measurable characteristics of absorbent structures include rewet and strike-through. Rewet is also described as "surface wetness". It is the amount of absorbed liquid that is detectable at the surface of an absorbent structure after absorption into the absorbent body. In measuring this characteristic, a structure absorbs a given amount of liquid, a given pressure is applied to the structure, and the amount of liquid detectable at the structure's liquid application surface is measured. An absorbent structure exhibiting "good" rewet characteristics would maintain a relatively dry surface, while a structure exhibiting "poor" rewet characteristics would transfer significant liquid back through to the surface of the structure.
Strike-through is the time measured for a given amount of liquid to pass through the facing of an absorbent structure and into its core. An absorbent structure exhibiting "good" strike-through characteristics would quickly accept and absorb applied liquids, while a structure exhibiting "poor" strike-through characteristics would allow applied liquids to form puddles on its surface.
Practitioners in absorbent structure technology have recognized that a positive density gradient, i.e., decreasing pore size with increasing depth into an absorbent structure, improves the performance of the structure. This allows liquids to be accepted into the structure in a region having large pore sizes. However, to improve rewet characteristics, decreasing pore size lower in the structure draws the fluid further into the structure, away from the wearer's skin. This provides a drier surface. It is recognized that decreasing the pore size of a hydrophilic material increases its capillary suction for aqueous liquids. This concept is explored in Meyer, U.S. Pat. No. 4,798,603; Cadieux, EP-A-0 359 501; and Kellenberger, U.S. Pat. No. 4,688,823.
Meyer teaches that the pore size should decrease in progressing from the topsheet to the transfer layer and to the core. In other words, there should be a negative pore size gradient or a positive density gradient. Such a configuration provides a capillary-pressure gradient between each layer, sucking fluid deeper into the absorbent structure, while preventing or reducing rewet.
Cadieux discloses a multiple layered absorbent structure which incorporates a positive density gradient from its cover sheet, through a transfer layer, and to and including a reservoir layer. The cover sheet is disclosed as a relatively low density, bulky, high loft nonwoven web material. The transfer layer may be composed of fibrous materials, such as wood pulp, polyester, rayon, flexible foam, etc., and the reservoir layer is a highly dense absorbent layer having a fine porosity such as compressed peat moss board.
Kellenberger attempts to achieve a concentration gradient in superabsorbent particles distributed within an absorbent fibrous mass or absorbent core. The superabsorbent may be distributed within the absorbent core in a number of concentration gradients: a positive concentration gradient, similar to Cadieux; a bi-nodal concentration gradient, having maxima proximate the top and bottom surfaces, and a minimum concentration at the center of the layer; and a distribution having minimal amounts of superabsorbent proximate the top and bottom surfaces, and a maximum concentration at the center of the layer. This absorbent core can be enclosed between two creped wadding or tissue sheets and used in an absorbent article further having a top cover sheet and a bottom barrier layer.
Other references have provided for multiple layers of absorbent materials in an absorbent structure. These references include Mesek, U.S. Pat. Nos. 4,670,011 and 4,960,477; Iskra, U.S. Pat. No. 4,605,402; Chen, U.S. Pat. No. 5,037,409; Ness, U.S. Pat. Nos. 4,842,594 and 4,880,419; Dawn, U.S. Pat. Nos. 4,338,371 and 4,411,660; and Allison, U.S. Pat. No. 4,531,945.
The prior art generally represents the advance of the absorbent structure art. However, continued advances in this art are needed. In particular, a new absorbent structure is needed which will quickly absorb body fluids, especially gushes of fluids, and strongly contain the absorbed fluids. Such a structure, if easily and economically manufactured, would be very useful in the manufacture of low cost disposable body fluid absorbent articles.